The present invention relates to an optical projection system, as well as to a method for using an optical projection system.
The invention particularly relates to an optical projection system of the type comprising a projection lens, illumination optics in front of the projection lens and at least one light modulator, such as a digital mirror device (DMD), comprising a plurality of reflective elements, wherein this system shows an optical axis and wherein said illumination optics allow to project light onto the light modulator and said light modulator allows to reflect the light in a controllable manner to different directions, such that ON state light cones, in which the light is projected onto the projection lens, as well as OFF state light cones can be formed.
In particular, the invention relates to an optical projection system which comprises a three-chip digital light processor (DLP).
The illumination system of optical projection systems of the above described type, in particular of three-chip DLP projectors, is straight forward. Light collected from a lamp is collected at the entrance of an integration rod and imaged from the exit of the rod via relay optics and a color prism on the light modulator, particularly on the digital mirror device. Since such optical system is telecentric, all the pixels on the light modulator, in other words, each of the aforementioned reflective elements, are illuminated with the same cone of light. In well-known devices having an F# number of 3.0, conventionally this is a cone with a half angle of 9.46xc2x0. The incidence angle of the chief ray of the cone of light is for a traditional F#3.0 device in the color prism around 20xc2x0.
Traditionally, such DMD itself is a micro electro mechanical system (MEMS) consisting of an array of mirrors which form the abovesaid reflective elements (for SXGA resolution 1280xc3x971024). Each individual mirror acts as a bi-stable device and can flip into two possible positions, usually between xe2x88x9210xc2x0 and +10xc2x0 with respect to a reference plane. When such mirror is tilted to xe2x88x9210xc2x0 (for F#3.0 devices) the chief ray is reflected over an angle of 20xc2x0, right into the projection lens. The hereby obtained state is called the ON state and the obtained light cone is called the ON state light cone. When the mirrors are tilted to +10xc2x0 (for F#3.0 devices) the chief ray is reflected over 60xc2x0, outside the projection lens. The hereby obtained state is called the OFF state and the respective light cones are called OFF state light cones. When light is falling in between the mirrors, the chief ray is reflected over an angle of 40xc2x0, resulting in a state which is called the FLAT state and the respective light cones are called FLAT state light cones.
In case the mirrors are in the ON state, the projection lens captures all the F#3.0 light cones leaving the mirrors. This means that all F#3.0 light cones pass the F#3.0 aperture of the projection lens.
In case the F#3.0 aperture is reshaped (e.g. a cat eye aperture), some of the light is blocked in the projection lens. Using a cat eye aperture results of course in a reduced light output, but can increase the contrast of the projector drastically, as explained in article SID 2002, Scott Dewald.
Another way to increase the contrast in DLP projectors is to increase the angle of the chief ray of the incident light. As a result the chief ray of the ON state light cone does no longer coincide with the optical axis of the system and is turned over a certain angle in respect to said optical axis. Also the FLAT state and OFF state cones are turned over a similar angle in respect to their normal positions. Tests have been shown that in this way the contrast can be increased, which probably can be explained by the fact that less diffused and diffracted light from the FLAT state light cone is captured in the projection lens.
Since also the ON state light cone is also shifted to bigger angles (with respect to the incident cone of light), the ON state light cone is not filling completely the traditional F#3.0 aperture of the projection lens. As a result, the light output is decreased to a quite large extent.
It is the object of the present invention to either increase the contrast level of the projection system with no or almost no loss of light output, or to increase the contrast tremendously with some loss of light output.
To this end, the present invention provides an optical projection system, comprising a projection lens, illumination optics in front of the projection lens showing a well-defined F# number, and at least one light modulator comprising a plurality of reflective elements, wherein this system shows an optical axis and wherein said illumination optics allow to project light onto the light modulator and said light modulator allows to reflect the light in a controllable manner to different directions, such that ON state light cones, in which light is projected onto the projection lens, as well as OFF state light cones can be formed, wherein said projection lens shows an F# number which is smaller than the F# number of said illumination optics, and wherein the chief ray of the incident light at the light modulator is chosen such that the direction of the chief rays of the obtained ON state light cones differ from the corresponding optical axis, so as to obtain an improved contrast.
By using a layout as described above, for the reasons explained before, an important increase of contrast can be obtained due to the fact that the chief rays of the obtained ON state light cones differ from the direction of the optical axis.
Meanwhile, by using a projection lens with an F# number which is smaller than the F# number of the illumination optics, a large portion of the light of the ON state light cones, if not all light of these ON state light cones, can be captured and no of almost no light output is lost.
In a preferred embodiment, the abovesaid technique is also combined with the use of a non-circular aperture, particularly a cat-eyed aperture. This of course reduces the light output (some light is blocked in the projection lens), but the contrast level will increase tremendously. The invention also relates to a method for using an optical projection system, said projection system comprising a projection lens, illumination optics in front of the projection lens showing a well-defined F# number, and at least one light modulator comprising a plurality of reflective elements, wherein this system shows an optical axis and wherein said illumination optics allow to project light onto the light modulator and said light modulator allows to reflect the light in a controllable manner to different directions, such that ON state light cones, in which light is projected onto the projection lens, as well as OFF state light cones can be formed, wherein said method comprises the steps of using a projection lens which shows an F# number which is smaller than the F# number of said illumination optics, and the step of choosing the chief ray of the incident light at the light modulator such that the direction of the chief rays of the obtained ON state light cones differ from the corresponding optical axis.
The invention can be applied in new projection systems, as well as in existing projection systems. Especially in the last case, the invention is of considerable importance, as an existing projector can easily be transformed to a projector having the features of the present invention, just by rearranging the optical components such that the direction of the incident light is changed and by replacing the projection lens by another one with a lower F# number.